1. Field of the Invention
The present invention relates to communications systems. More specifically, the present invention relates to communications systems operating over fading channels.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Mobile satellite communication systems are typified by the movement of a receiver relative to a transmitter or vice versa. The communications link between transmitters and receivers in mobile communications systems is characterized as a fading channel. Mobile satellite communications systems, having a transponder on a spacecraft and a transceiver on a ground based vehicle, cellular telephone systems and terrestrial microwave systems are examples of communications systems operating over a fading channel. A fading channel is a channel which may be severely degraded due to numerous effects including multipath fading, doppler shifts, doppler spreads and additive noise. In the often used L band, large doppler frequency spreads and shifts due to vehicle movement are particularly problematic.
The conventional solution to the problem of doppler shifts and spreads for mobile communication systems operating at L band was to use pilot tones and coherent demodulation. (See "Dual Pilot Tone Calibration Technique (DPTCT)," by M. K. Simon in IEEE Transactions on Vehicular Technology, vol. VT-35, no. 2, May 1986, pp. 63-70.) A pilot tone is a signal of a discrete frequency in the spectrum within the data band which is transmitted with the data, demodulated and used to recover the data. As is known in the art, the ideal place for the pilot tone is in the center of the data band. Unfortunately, a pilot tone in the center of the data band would interfere with the data. Accordingly, it was necessary to shape the data spectrum to provide a notch for the pilot tone. There are at least three disadvantages with this approach. First it required the reshaping of the data spectrum, adding the cost and complexity of the system. Second, it necessitated an expansion of the bandwidth of the system. This is particularly problematic in mobile satellite communication systems which are typically severely limited in bandwidth and power. Finally, the need for guard bands around the pilot tone, in the notch, amounts to a waste of available bandwidth.
These problems are ostensibly addressed somewhat by dual tone systems which provide discrete carrier tones at the edges of the data band which are beat together in the receiver to provide a tone in the middle of the band. The problems with these systems is that when multiple channels of data are transmitted, there is a pilot tone between each of the data bands. This presents the potential for adjacent channel interference. In addition, the need for guard bands around the pilot tones, also causes some bandwidth expansion.
Thus, there is a need in the art for a mobile communication system which addresses the problem of doppler shifts and spreads while conserving bandwidth and without impairing the performance or imposing significant additional power requirements on the system.